This invention relates to hermetic compressors. More particularly, the field of the invention is that of crankcase heaters for low pressure hermetic compressors which prevent the migration of refrigerant from the evaporator side of a refrigeration system to the compressor.
During routine operation of a refrigeration system, the compressor is intermittently shut down by a thermostat. When the compressor is thermostatically shut down, and also when the refrigeration system is de-energized, the compressor ambient temperature decreases. Gaseous refrigerant from the evaporator or accumulator can migrate through the suction line and into the compressor where the lower temperature condenses the refrigerant. The presence of liquid refrigerant in the compressor on startup can cause liquid slugging, which may rupture the compressor gaskets, damage bearings, and the like due to the resulting high pressures.
One type of prior art compressor includes a solid state heater or the like and is in thermal contact with the compressor housing or the oil and refrigerant in the sump. The heater helps to prevent the problem of liquid refrigerant migrating from the evaporator, traveling through the suction line, and entering the compressor. Further, compressor crankcase heaters have been installed to decrease the temperature difference between the evaporator side and the compressor ambient temperatures, and given a sufficient period of time the crankcase heater reduces liquid migration.
One benefit of a crankcase heater is the ability to boil off liquid refrigerant in the oil sump. One such crankcase heater is disclosed in U.S. Pat. No. 4,755,657, which includes a PTC heater mounted in a shallow depression at the bottom of the compressor housing. However, this design has several disadvantages. One disadvantage involves the shallow depression which is a required modification to the compressor housing. Such a modification may not be desirable for the housing, and the heater cannot be used with already existing compressors not having such a depression. Another disadvantage is that the system must have some means for attaching the PTC heater to the compressor.
A need still exists for a crankcase heater which is efficient to produce, convenient to install, effective in operation, and which does not require alteration of the compressor housing.